the transition from shallow to deep convection over land: a diurnal-cycle analysis of arm sgp...
DESCRIPTION
What determines whether daytime shallow convection remains shallow or transitions to late-afternoon deep convection? ? Main Result: Humidity slightly above the PBL appears to be most important factor controlling whether or not the transition takes place; other factors (CAPE, CIN, boundary layer heterogeneity, surface fluxes) not as important Differences with transition over ocean: (a) shorter time-scale, (b) no congestus phase, (c) moisture above PBL from large-scale advection, not moistening by shallow convection Plans: We are now constructing “composite forcing” SCM/CRM cases for the days that shallow convection transition – We are looking for collaborators! Process-level observations: Great potential for vertically-pointing and 3-D cloud / precipitation radars to observe cloud sizes, orientation, vertical velocities, downdrafts/cold poolsTRANSCRIPT
The Transition from Shallow to Deep Convection over Land:
A Diurnal-cycle Analysis of ARM SGP Observations
Stephen A. Klein and Yunyan Zhang (LLNL)October 31, 2012
ASR Fall Working Group Meeting
Zhang Y. and S. A. Klein, 2010: Mechanisms affecting the transition from shallow to deep convection over land: Inferences from observations of the diurnal cycle collected at the ARM Southern Great Plains site. J. Atmos. Sci., 67, 2943–2959.
Zhang, Y. and S. A. Klein, 2012: Factors controlling the vertical extent of fair-weather shallow cumulus clouds over land: Investigation of diurnal-cycle observations collected at the ARM Southern Great Plains site. J. Atmos. Sci., submitted.
What determines whether daytime shallow convection remains shallow or transitions to
late-afternoon deep convection?
• Main Result: Humidity slightly above the PBL appears to be most important factor controlling whether or not the transition takes place; other factors (CAPE, CIN, boundary layer heterogeneity, surface fluxes) not as important
• Differences with transition over ocean: (a) shorter time-scale, (b) no congestus phase, (c) moisture above PBL from large-scale advection, not moistening by shallow convection
• Plans: We are now constructing “composite forcing” SCM/CRM cases for the days that shallow convection transition – We are looking for collaborators!
• Process-level observations: Great potential for vertically-pointing and 3-D cloud / precipitation radars to observe cloud sizes, orientation, vertical velocities, downdrafts/cold pools
?
Days with locally-generated convection coupled to the surface & PBL
We determine which environmental parameters control the regime by– Contrasting parameters between days with different regimes– Examining co-variability of parameters with precipitation characteristics on days
with late-afternoon deep convection
40 days 40 days 70 days
PRECIPITATION
CLO
UD
Humidity plays a dominant role
• Higher moisture in the layer 2 – 4 km above ground is present on days with late-afternoon deep convection
• Extra moisture in this layer comes from deeper southerly flow, not moistening by local convection
• Higher moisture in this layer promotes an earlier onset time to deep convection
at 1130 LST2 – 4 km